D. Van Norren

Spectral reflectance of the human eye

Spectral reflectance of the eye was assessed in four young Caucasian subjects with the Utrecht densitometer. Three retinal locations were studied, the fovea, the optic disk and a spot 12 deg temporal on the horizontal meridian. Reflectance was measured with the visual pigments bleached away. The measuring field subtended 2.5 deg. The reflectance factor, relative to an artificial eye with a focal distance of 21.3 mm, was found to be lowest in the blue (0.1% for 419 nm) and highest in the red (10% for 711 nm). A model with six parameters, four densities of (non-photolabile) ocular pigments and two reflectance factors, was proposed to explain the experimental findings. A good fit to the data was obtained and the calculated densities of the ocular pigments came close to data found in the literature.

Intensity and polarization of light scattered at small angles from the human fovea

Abstract We have investigated the equivalent reflectance and the degree or preservation or polarization of light scattered at the human fundus in vivo . Measurements are performed at various angles of incidence and reflectance with both a bleached and unbleached state of the visual pigment. In the light that is scattered from the fovea two components were distinguished. First, a wide angle scattered component whose behavior in terms of equivalent reflectance and degree of polarization is independent of the angle of incidence and reflectance. Also, its reflectance is only moderately affected by the density of the visual pigment. Second, a directional component that is most prominent in a bleached state of the visual pigment and with a central position of the entrance and exit lightpath. The degree of preservation of polarization is about 90% at short wavelengths and decreases to about 40% for red light. Polarization is almost invariant for the angle of incidence and reflectance and the density of the visual pigment. To explain the results a model is proposed in which the scattering of both components is assumed to take place at the same layer. A consequence of this model is that a significant amount of light leaks between the receptor outer segments.

Endoillumination during vitrectomy and phototoxicity thresholds

AIM To assess the retinal phototoxicity hazards of and to provide safety margins for endoillumination during vitrectomy. METHODS The absolute power and spectral distribution from various light sources and filter combinations that are commercially available for vitreous surgery were measured. The maximal exposure times based on the ICNIRP safety guidelines for photochemical and thermal injury of the aphakic eye were calculated. Additionally, the effect of various measures that reduce the risk of phototoxicity was evaluated. RESULTS Measurements of the spectrum and energy indicated that the ICNIRP safety guidelines for photochemical retinal damage are exceeded within 1 minute for nine out of 10 combinations tested. With an additional 475 nm long pass filter, light levels below 10 mW, and a distance from light probe to retina of at least 10 mm, the allowable exposure time can be increased up to 13 minutes. Thermal damage can be anticipated when the light probe touches the retina. CONCLUSION Commercially available light sources for endoillumination during vitrectomy are not safe with respect to photochemical retinal damage. Even with maximal precautions macular phototoxic damage remains a factual danger during vitrectomy.

Imaging retinal densitometry with a confocal Scanning Laser Ophthalmoscope.

Abstract We describe a novel use of the Scanning Laser Ophthalmoscope (SLO), viz. as an imaging retinal densitometer. In our SLO a helium-neon or an argon laser beam is moved in a raster pattern over the retina; the reflected light is descanned (confocal SLO) and collected by a photomultiplier. Images of the fundus subtending 22 by 18 deg are displayed on a TV monitor. Single frames taken with 514 nm light were stored in a computer in arrays of 256 by 256 pixels and density differences between dark adapted and bleached images were calculated. With a full bleach density differences of about 0.35 were found in the center of the fovea; at retinal eccentricities of 15–20 deg we found 0.15. After selective bleaching with 633 nm light substantial density differences were only seen in the foveal area. We conclude that the confocal SLO is a very suitable instrument for imaging fundus reflectometry.

Local photoreceptor alignment measured with a scanning laser ophthalmoscope.

The aim of this study was to develop a fast test for local photoreceptor alignment. Photoreceptor alignment is an important indicator of retinal integrity. Digitized images of fundus reflectance were obtained for 20-30 pupil entry positions with a custom built scanning laser ophthalmoscope (SLO). The data permitted the calculation of curve peakedness of the optical Stiles-Crawford effect (SCE) as a function of retinal location. We found that the peakedness is low in the central 0.5 deg, reaches a maximum at an eccentricity of 1-2 deg and gradually drops with increasing eccentricity. These data are in conformity with the anatomy of foveal cones. Additionally, the psychophysical SCE was measured with red light and an 8 deg stimulus. The mean peak position of the SCE in the pupil plane for both methods was similar, but the optical SCE was clearly steeper. The SLO provides a fast, reliable and objective way to determine local receptor alignment in the central retina.

Cone spectral sensitivity and chromatic adaptation as revealed by human flicker-electroretinography

Abstract The human ERG response to a 40 Hz stimulus was measured using a synchronous detection technique (lock-in amplifier). Thus it was possible to record spectral sensitivity quickly and easily. Check experiments showed that only the cones contributed to the total response. Adaptation to a red background of3.7 × 104 trolands, and a blue background of2.2 × 104 trolands caused selective depression of spectral sensitivity. Neither green nor white adaptation altered the spectral sensitivity. The results of parallel experiments on a protanope indicated that no change in spectral sensitivity took place during his exposure to intense coloured backgrounds. Measurements of the influence of chromatic adaptation were also performed using a psychophysical threshold criterion for sensitivity. The results are in close agreement with the ERG data. The recovery of the response after exposure to coloured backgrounds of various intensities indicated that the selective depression of the luminous spectral sensitivity can be due to both neural adaptation and to bleaching of the photopigments.

Foveal cone mosaic and visual pigment density in dichromats.

1. Optical reflectance spectra of the fovea were measured in ten subjects with normal colour vision, ten protanopes and seven deuteranopes. Four conditions were used: perpendicular and oblique angle of incident and reflected light on the retina, both in a dark‐adapted and a fully bleached state. 2. The spectra were analysed to assess the effects of dichromacy on the cone mosaic. A replacement model, i.e. one where the total number of cones remains unchanged and all cones are filled with a single type of pigment, was found to fit our data best. 3. The analysis of the spectral fundus reflectance also provided estimates for densities of photo‐labile and photo‐stable retinal pigments and fraction of long wavelength‐sensitive (LWS) cones. Visual pigment density was 0.39 for protanopes and 0.42 for deuteranopes, significantly lower than the 0.57 found for colour normals. Macular pigment density was 0.54 for colour normals, 0.46 for protanopes and 0.42 for deuteranopes. 4. For colour normals the LWS cone fraction was 0.56, in agreement with psychophysical literature. The LWS cone fraction for protanopes was ‐0.04, and for deuteranopes 0.96, consistent with their Rayleigh matches.

Photoreceptor function in unilateral amblyopia

We investigated whether photoreceptor function in amblyopic eyes differed from that in non-amblyopic eyes. Photoreceptor function was assessed with the optical Stiles-Crawford effect (SCE), psychophysical SCE, and foveal visual pigment density in both eyes of ten unilateral amblyopic subjects. Optical SCE and density measurements were carried out with a custom-built scanning laser ophthalmoscope (SLO). Amblyopic and normal eyes did not differ in Stiles-Crawford effect, nor in foveal visual pigment density. Contrary to suggestions in the literature, we found no indication of retinal dysfunction at the level of the cone photoreceptors in amblyopic eyes.

Transient tritanopia at the level of the ERGb-wave

Abstract A blue stimulus that is just visible in the presence of a yellow adapting field remains invisible for several seconds after the yellow field has been turned off. This “transient tritanopia” is shown to exist already at the level of the b-wave of the electroretinogram. The experiments were done on rhesus monkeys and the results were qualitatively confirmed in human subjects with psychophysical and ERG methods.

Wavelength dependence of reflectometric cone photoreceptor directionality

We present evidence for the wavelength dependence of the directionality of light reflected from cone receptor cells (optical Stiles-Crawford effect): Blue light is more directional than red. According to the waveguide-scattering model of Marcos et al. [J. Opt. Soc. Am. A 15, 2012 (1998)], directionality is the sum of a waveguide component and a scattering component. The latter is proportional to 1 over wavelength squared, and it is related to the row-to-row spacing of the cone lattice. Our results allow a firm confirmation of Marcos et al.s theory. For a 1.9-deg foveal area, group mean (n = 18) cone spacing was 3.42 microm, in good agreement with anatomical data. Group mean waveguide directionality was 0.077 mm(-2).